This invention relates to turbomolecular pumps, and, more particularly, to a turbomolecular pump of the type suitable for use with equipment requiring ultrahigh vacua.
Generally, a turbomolecular pump is used with equipment requiring vacua, such as nuclear fusion devices, electron microscopes, etc, with the vacua obtained in turbomolecular pumps nowadays reaching as high as 10.sup.-11 Torr. In addition to higher vacua, so-called clean vacua have, in recent years, been in demand in which there is no residual gas of hydrocarbons, such as oil vapor.
The principle of the operations of turbomolecular pump is that the higher the molecular weight of a gas, the greater is the compression ratio thereof. Thus, a turbomolecular pump is able to readily exhaust oil vapor, making it possible to obtain clear vacua. However, a disadvantage of turbomolecular pumps using oil-lubricated ball bearings for journaling a rotor resides in the fact that, although the vacuum container is free from contamination with oil vapor during operation, diffusion in reverse of the vapor of lubricating oil occurs when the pump operation stops, so as to cause contamination of the vacuum container. To avoid this disadvantage, various proposals have been made to provide improvements in turbomolecular pumps, such as, for example U.S. Pat. No. 4,023,920, which provides a turbomolecular pump using controlled electromagnetic bearings.
However, demand for ultrahigh vacua has increased in recent years, and to meet this demand it is necessary to minimize gas releases from surfaces of parts of not only a vacuum chamber but also the pump itself. To this end, it is usual practice to rely on "baking" to release gas by heating the parts at high temperature when the pump is operated in ultrahigh vacua. More specifically, baking is performed to raise the temperature of the surfaces of the parts exposed to ultrahigh vacua to a high level to allow the gas that has been absorbed by the parts to be released as much as possible, before the pump as a whole is restored to a room temperature condition to obtain high vacuum therein. The effects achieved by baking can be increased by raising the temperature to a higher level. Thus, it would follow that it is very effective to raise the allowable temperature for performing baking in order to obtain higher vacua.
However there is some difficulty in raising the baking temperature as high as would be desired. In a turbomolecular pump now available, bearings for journaling a rotor are mounted on the high vacuum side or the suction side, and the bearings used for this purpose generally comprise ball bearings with dampers and active magnetic bearings with coils. The bearing mechanism of these types have an allowable temperature range of between about 100.degree. and 120.degree. C., because the ball bearings have their allowable temperatures limited by the allowable temperatures for the lubricating oil and the bearings themselves and the magnetic bearings by the temperature range to which the sheath of the winding is resistant. Such being the case, in the turbomolecular pumps now available, the baking temperature is restricted to a level below 120.degree. C. which is the upper limit of the baking temperature range for parts around the suction port of the pump. As a result, it is difficult to achieve higher vacuum in ultrahigh vacua because there are limits placed on the allowable temperature range for baking.
The aim underlying the present invention essentially resides in providing a turbomolecular pump capable of avoiding the disadvantages encountered in the prior art while also achieving higher vacua than the prior art.
An object of the present invention is to provide a turbomolecular pump capable of achieving clean vacua not only when the pump is operative but also when it is inoperative.
In accordance with the present invention a turbomolecular pump is provided which comprises stationary blades axially arranged in a casing in a plurality of stages, and movable blades located between the stationary blades and mounted at an outer periphery of a rotor located in a center of the casing, with bearing means being provided for supporting the rotor at both a high vacuum end and a low vacuum end. The bearing means at the high vacuum end of the rotor includes a magnetic bearing of attraction type comprising a first permanent magnet of the fixed type having a characteristic such that the Curie point of the material is high, and a second permanent magnet located in juxtaposed relation to the first permanent magnet.
Additional and other objects, features and advantages of the invention will become apparent from the description set forth hereinafter when considered in conjunction with the accompanying drawings.